Level gages



Sept. 17, 1963 p, J. GARDNER LEVEL GAGES Filed July 13, 1961 FIG. l

UNITS OF LIQUID LEVEL' l lll/IIA f-zo INVENTOR R. E m m n @M J. m L 4T MA P Y B m F United States Patent O 3,1tl3,815 t Lili/EL @AGES l Paul Si.Gardner, Davenport, iowa, assigner to The Bendix Corporation, .lavenpcrnIowa, a corporation of Delaware Filed July 13, 1961, Ser. No. 123,80 4Claims. y(Cl. 7S-304) This invention relates to improvements in levelgages and more particularly to capacitance level 4gages tor use' inmeasuring the level of liquids whosetemperature difhereinafter appear.

"In the drawings:

n FIG-L is a graph of the relation between capacitance of a gageIan'dlliquid level in different circumstancesg.

FIG. 2 is a cross-sectional view of the level gage em,

vbodying the invention, taken on -line 2-2 of FIG. 3; and

FIG. 3 is a cross-sectiorlview taken on the meandering line 3 3` of FIG.2.

To illustrate the problem solved by the invention, let it be assumedthat it is required to fill a vessel having the tempera-ture of theambient atmosphere with liquid hydrogen at minus 423 degrees FahrenheitandV then to withdraw the liquid, knowing at all times the level of theliquid within the vessel. Assume too that the level is to be determinedwith a capacitance gage extending over the depth of the vessel coupledwith means for measuring the capacitance of th-e gage (or the effect of4capacitance change). The dielectric constant of the liquid beingmaterially greater than the vapors ofthe ullage space above ,the liquid,the capacitance of the gage will increase as liquid level increases. l

Capacitance of the gage will also Vary as the area and spacing of thegage electrodes are altered as an incident to temperature change.problem in the design of a gage if they occurred nearly instantaneous-lyor if the rate of lling |and emptying of; the vessel was much slowerthan theirate at which t-he temperature of the gage elements could bechanged. In

practice, however, successive increments along the length of the gageare cooled and contracted as the liquid level rises higher in thevessel. If the relative rate of gage cooling and vessel filling are suchthat sections of the gage length continue to contract after immersion inthe liquid, then tor a given filling rate thel curve of capacitance ofthe gage plotted against liquid level might be like the dashed curve l@in FIG. l. When the vessel has been filled the gage elementsV wil-l becooled to substan Suchy changes would present no,

any given level. The latter is determined by the differl enceV betweencooling rate and filling rate-a factor termed thermal lag. herein. Itisan object of the invention to provide a gage in which these two curvesrnore nearly coincide over a greater range of lling rates.

` Gages made in accordance with the invention include, as features,elements which conduct heat readily whereby their cooling andcontraction tends to occur above the gas-liquid interface during fillingand they comprise means for decreasing the spacing between gageelectrodes as the electrode areas decrease, where electrode area meansthe effective areas of the electrodes between which the electrostaticlield extends.

The dielectric constant of the liquid being greater than the gas, thechange in gage capacitance. incident to electrode area and spacingchange is less if these changes 1 occur in gas than 1in liquid. Thus theproblem is minimized by use of gage elements, including the mountingmeans Ialong the gage length, which conducts heat readily. In addition,the mounting means is arranged so that most of its mass is divorced fromthe function of separating the gage electrodes.

Gage capacitance decreases as electrode area is decreased and itincreases as electrode spacing is decreased. In gages made according tothe invention the mounting means is arranged so that spacing isdecreased rather than increased when the electrodes contract, and in onepre-.- ferred form of the invention the mounting means 'effectrelativemovement of the electrodes in a direction tending to overcome reductionin their effective area incident to contraction.

These features are incorporated in the gage illustrated in FIGS. l and2. This device comprises a pair of electrodes separated by spacers freeto movewith the electrodes and means to insure that the electrodesremain in engagement with the spacers as the electrodes and spacerschange dimension with temperature. Means are included torv mounting theelectrodes such that dimensional changes with temperature of themounting means does not alter electrode spacing. In the embodimentselected for illustration in FIGS. 2 and 3 one electrode has portionsdisposed within the cross-sectional area bounded,

but not encompassed, by the other. Accordingly, as the electrodes andspacers contract, the electrode spacing is diminished and, whereascontraction of the electrodes reduces their effective area, thisreduction is diminished by the Vamount of spacer contraction.

Y In FIGS. 2 and 3, the electrodes are numbered 14 and 16.Advlantageously, as shown, they are formed of aluminum extrusions 'forhigh heat conductivity, and 'good dimensional unitormity. Electrode 14comprises a backing plate rectangular in cross section and three similarexpand until after the level :falls below them. Thusthe vcapacitance ofthat gage segment which lies just below tending ribs.

ribs extending Yin parallel from one side of the plate, one at each edgeand the third equidistant from and between the others. Electrode 16 issimilar comprising a rectangular plate and four equally spacedsidewardly ex-.

The basic dimensions and spacing'of all.

ribs lare the same in this example. The electrodes are arranged so thatthe three ribs of electrode 14 are disposed-in the three spaces betweenthe rour ribsV of elec trode 16. The electrodes do not touch one anotherbut are held apart by three small, thin spacers 18 which are notched atone -end to receive the ends of the ribs of electrode 14 and `abutagain-st the backing plate and the roots of the ribs ot electrode 16 attheir opposite ends. Electrode 14 is secured by any convenient means,'such as machine screws 20, to one side 22. of 'an electricallyinsulating mounting bar 24. A spring clip Zdholds electrode 16 tightagainst spacers 18 and the spacers tightly against the ribs ot electrode14. The spring clip 26 is fastened at lits ends by lany convenientmeans, such as Ascrews 28, to side 22 of the mounting b-ar 24 tromalessia whence it extends along the sides of the endV ribs-of electrode16 to the rear of the backingplate of electrode,

26 Whereitbears against the 'backing plate. The assembly thus fardescribed is disposed within an elongated still well 34 andjis attachedthe-reto by suitable tasteners 3i? which kextend through the well andinto the mounting f bar 24.' V The well istpreferably formed of aconductor snch .as aluminum tubing The still well serves as `a means formounting the gage inits 'vessel and it isolates the capacitor from otherelectrostatic lields within the vessel. It is connected electrically toground. In addition, the stillw-ell is open at its lower end,perforated, or' otherwise arranged, so that owell'-t 4andthe outerelectrode is permitted in cases where one capacitor electrode isgrounded, permits mounting the spring clip 26 onthez well 34 rather thanon the mounting bar 24 if that -is more convenient. course the springhowever'V mounted may haveany of ra wide varietyof shapes. The springyform shown. is iadvantageous is very long gages `because it presents noproblem in inserting Iand assembling the gage into. the well. V y

In operation of the gage shown, assumingthatr it is mounted in a vesselto be iilled with a cryogenic liquid, as the level rof .liquid risesinthe vessel and still well, heat is withdrawn rapidly from the stillwell and electrodes whereby contraction of successive sections of theseelements tends to be completed prior to their immersion in liquid. Asthe liquid level approaches a mounting assembly, the mounting bar 24,fasteners 2d, 28, `and 313,`

the spring 26z|and spacers 18 lgive up heat `to the still well andelectrodes. Except forY themounting bar 24, these elements andespecially spacers 18, have very small mass and so are substantiallyfully cooled @and contracted prior to `immersion in the liquid even atrelatively high filling l rates. The spacers have goed thermal contactwith both electrodes. Itis irnportantthat spacers 18 cool and con-Atract napidly so that spring 26 can urge the electrodes 14 andY 16together' to their cold position prior to submersion in the liquid.

The mounting .ber 24 has'appreciable mass and accordingly is not sorapidly cooled and contracted.y However, dimensional changes in the bardo not alter the electrode spacing orV effective areav soit is notmaterial kif contraction of themounting .bar is not complete .untilafter immersion. This is true as wellv of the vother xnount-r ingelements 26, Ztl, 28,1and 30. Among the mouting elements expansion andcontraction of the spacer elements alone areefl'eotive to control,electrode` spacing. n No problem is presented during withdrawal of theliquid from the container rst because in most instances good heat OfyThe spacer material should give up its heat readilyso that the spacersIare cooled prior toimmersion. ln practice the mass of thespacers issmall whereby they are easily cooled and material selection is notverycritical.

Triiluorochloroethylene land similar materials `are recom,` lv.

mended. l l

The combination lof the spacers and electrode shape i provide specialadvantages and permit the construction of precision gages in very longlengths. The electrode shape permits manufacture by extrusion andresults in members having very uniform dimensions. yIn addition, thearrangement of backing member, and ribs permits veryac curate machiningwith conventional machinetools whereby -therib depth along the length ofan element may be precisely contoured to change incremental capacitancelengthwise of the gage. Such contouring is desirable when the vessel inwhich it will be installed is not uniform throughout its length and" itsis desired that the capacitance of the gage vary as a predeterminedfunction, such tor example as linearly with liquid level or volume orout altering the mounting arrangement or the basic electroderelationship..` Such contouring is illustrated at '40 and 4Z in FIG. 3.The contour at 42 compensates ier the slight increase in capacitance atthis point resulting from the fact that the'dielectric constant of thespacer ,f

is greater than unity.

Iclaimz` Y l. A `cat nacitancelevel gage comprising a Ipair of elongatedelectrodes'one being dispose-d partly within a cross f sectional arcabetween portions of the other electrode, a

plurality of electrically insulating spacers disposed between sa-idelec-trodes Aand having a dimension to limit the degree in'which saidVone electrode may be partly dis-` there is no substantial heat input toWarrn the gage and second lbecause Veven -ifheat was available sectionsof the gage Would`not be warmed until after the liquid level had recededbelow them.

A wide variety of material is suitable 'for use in lrnak-` ing thespacers and mounting bar. If, as in the particularl embodiment shown,thespacer connects the still well and` that one of the electrodesvwhich'm-ust be insulated from the still well, then the bar, like thespacers, must be an electrieal'insullator. Many ofthe 'plastic materialsare suitable. n Advantageously Vthe mounting 'bar Yis a relatively poorconductor ot heat so that heattransfer from the bar to the electrodev.just prior to immersion of the portion of the electrode it contacts islimi-ted. VOnce immersed most of its heat will be lost to the liquidrather Ythan the electrode.- Most plastics including trilorochloro-vethylene and tetraflnoroethylene laresuitable.

posed withinksaid area, and mounting means for saidy electrodesincluding means for urging said electrodes into engagement withsaidspacers and for increasing the degree in which said one electrode isdisposed within` said area upon contraction of said spacers to reducesaid dimension as an Vincident to temperature reduction.

2. A capacitance level gage comprising, a pair of elongated electrodes,one having a portion along its length f extending partly within` a crosssectional area between spaced portions of the other, at least oneelectrically insulating spacer interposed between said electrodes andlimiting -by its dimension the degree in which said one electrode may,extendv within said area, an elongated still wellA surrounding saidelectrodes, means for ixedly mountf ing either one of said electrodes tosaid. still well, rand resilient means interposed between said stillwell and that one of said relectrodes not so xedly mounted and bearingthereon in the direction'of said dimension with a force suicient toVeffect engagement of both electrodes Iwith said spacer despite.contraction and expansion of said electrodes and said spacer asangincident to temperaturel change. j *Y 3. The invention defined inclaim 2 in which said Vstill 1 well is formed of' aluminum and saidelectrodes are formed as extrusionsof aluminum. 'Y

4..A capacitance level gage comprising an elongated still well and apair of electrodes each having a backing plate and ribs extendingtherefrom, the ribs of a rst y -oneiof said electrodes being disposed inthe spaces between the ribs of the second one of said electrodes, and

the electrodes thus arranged kbeing disposed with-in the Y stillwelLmounting means for the electrodes comprising at least oneeleetricallyfinsulatingV spacer'interposed between -the backing plate ofone of said electrodesand the ribs. of the other of said electrodes'andfurtherfeomprisling an electrically insulating mounting'elementsecuredbetween one ofsaid electrodes and said still well and resilient meansinterposed between the other of said electrodes and thefstructureformedby the combination of 5 the still Well, mounting element and the oneelectrode, References Cited in the le of this patent said resilientmeans urging said electrodes together in a direction and with a forcesucient to eect engagement UNITED STATES PATENTS of both of saidelectrodes with said spacer despite dimensional change ofthe electrodesand spacer in the direction 5 1115 of separation of the backing plate ofSaid yone of said elec- 2691223 obl l, O t '12 1954 trodes and the ribstof said other of said electrodes ineier m c dent to temperature change.

4. A CAPACITANCE LEVEL GAGE COMPRISING AN ELONGATED STILL WELL AND APAIR OF ELECTRODES EACH HAVING A BACKING PLATE AND RIBS EXTENDINGTHEREFROM, THE RIBS OF A FIRST ONE OF SAID ELECTRODES BEING DISPOSED INTHE SPACES BETWEEN THE RIBS OF THE SECOND ONE OF SAID ELECTRODES, ANDTHE ELECTRODES THUS ARRANGED BEING DISPOSED WITHIN THE STILL WELL,MOUNTING MEANS FOR THE ELECTRODES COMPRISING AT LEAST ONE ELECTRICALLYINSULATING SPACER INTERPOSED BETWEEN THE BACKING PLATE OF ONE OF SAIDELECTRODES AND THE RIBS OF THE OTHER OF SAID ELECTRODES AND FURTHERCOMPRISING AN ELECTRICALLY INSULATING MOUNTING ELEMENT SECURED BETWEENONE OF SAID ELECTRODES AND SAID STILL WELL AND RESILIENT MEANSINTERPOSED BETWEEN THE OTHER OF SAID ELECTRODES AND THE STRUCTURE FORMEDBY THE COMBINATION OF